1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor and particularly to a magnetostrictive torque sensor including a detection coil for detecting a magnetic property change of a magnetostrictive film to detect a torque.
2. Description of the Related Art
In an electric power steering device installed in a vehicle, a torque sensor detects a torque which is applied to a steering shaft by driver's steering. Thus, steering assist power is provided depending on torque signals output from the torque sensor. A magnetostrictive torque sensor is disclosed as such a torque sensor (See JP2004-239652A, FIG. 4 and JP2005-331453A, FIGS. 11 and 12). In this technology, a magnetostrictive film having magnetic anisotropy is attached on a surface of the shaft. When a torque is applied to the shaft from the outside, a magnetic permeability change in the magnetostrictive film corresponding to twisting force is detected as a change in impedance regarding a detection coil. Moreover, in technologies disclosed in JP2005-331453A, a pulse of excitation voltage is applied so that the change in the impedance of the detection coil is detected. In the technologies, the pulse of the excitation voltage is applied with a switching circuit so that a maximum of increasing current which flows in the detection coil is limited within a predetermined range.